Will it stop working ?
Yes, it will stop working. Catalina and higher do not support 32-bit software. To use CS5 you need to keep the same hardware and macOS. Make backups, as computers do not last forever.
"Are legacy/perpetual versions of Photoshop, compatible with macOS Monterey?"
"No, legacy/perpetual versions of Photoshop were not designed or tested to work on macOS 12.0 (Monterey). They are not supported in any way for use on macOS Monterey."
will a day come, when I might be'forced' to upgrade from my High Sierra?
That's inevitable. It'll probably happen when your hard drive fails or you have some other hardware issue.
Provided you can run a system with nothing later than the OSX Mojave operating system then you can keep running CS5.1. Any operating system after that (Catalina and later) will not run CS5.1.
One other option available to you is to dual boot with CS5.1 running on Mojave on an external hard drive.
Other than that, new Macs (particularly M1s) will require an Adobe Cloud subscription.
if I were to upgrade from Hi Sierra to Mohave.
Did I miss that you asked about Mojave before Terry? If so, I apologize. I thought you had only mentioned Monterey (macOS12) and Big Sur (macOS11) and that I had answered that CS5 won't run on Catalina (macOS10.15) and higher.
Mojave is macOS 10.14 and therefore is fine.
if I were to upgrade from Hi Sierra to Mohave
Is your Mac offering an upgrade to Mojave (mentioned in your latest post) or Monterey (mentioned in your first post)?
Mojave (a superseded operating system released in 2018) is the last operating system on which you can install and run CS5.1. You're unlikely to see any improvement in upgrading from High Sierra to Mojave and it won't change how you use CS5.1.
Monterey is the current Mac operating system. You cannot install CS5.1 on Monterey.
...will a day come, when I might be 'forced' to upgrade from my High Sierra?
You might be OK for as many years as you keep using your current Mac. But the day you decide you want a new Mac, or if your current Mac stops working, that is when you can’t stay on an older version of macOS, because the latest Macs run only macOS 12 or later. If you buy a new Mac, you can’t continue using Photoshop CS5, because neither it nor High Sierra runs on it.
If your current Mac dies, the only way to keep using High Sierra and Photoshop CS5 is to buy a used Mac old enough to run them.